Production of cellulose esters



fiatented File-pt. 9,

UNITED STATES PATNT" OFFICE Alvah Dale Clark and William F. Reichert,Meadville, Pa., assignors to American Viscose Corporation, Wilmington,Del., a corporation of Delaware No Drawing. Application December 6,1944, Serial No. 566,952

4 Claims. (Cl. 260227) 1 2 i This invention relates to improvements inthe the result that it is extremely difiicult to obtain production oforganic esters of cellulose. a product having the same average acylcontent The production of cellulose esters by the sothroughout, and thefinal product is not homogcalled homogenous process (solution method) inenous. Articles obtained from such esters, as

volves esterification of the cellulose by treatment for instanceartificial fibers, films, or the like, do thereof with an organic acidanhydride, in the not possess uniform properties and are of genpresenceof a catalyst and an organic diluent or erally inferior quality.

solvent for the ester being formed. There is ob- The present inventionprovides a method wheretained, thus, a homogenous viscous solution of byan increase in temperature of the primary an ester which issubstantially a tri-ester. In acylation solution is avoided, and, thetemperaorder to hydrolyze or saponify the tri-ester to a ture ismaintained at substantially not in excess more lowly esterified product,which is of greater of about 30 0. throughout the conversion of thecommercial importance, water is added to the triexcess anhydride to thecorresponding acid, and ester solution, commonly termed the primarysubsequent hydrolysis of the primary ester in acylation solution, inexces of the quantity.resolution.

quired to convert remaining acid anhydride to the We have found thatmore accuratecontrol of corresponding acid, and the solution ispermitted the hydrolysis of the primary ester, and hence a to stand orripen until the acyl content of the more homogenous final product, areobtained if, ester is decreased to the desired extent. Thereinstead ofwater in the liquid phase, sufiicient ice t r, w r /or ot er non-Solventfor the is added to the primary acylation solution to cellulose is addedin an amount suificient to preprovide enough water to kill the excessacetic 'cipitate the cellulose ester from solution. anhydride presenttherein, and efiect hydrolysis Although proposals have been advancedwhereof the tri-ester.. The ice may be added in the by both theesterification and hydrolysis reacform of fine shavings or coarserflakes. The heat tions are effected at higher temperatures,experigenerated by the reaction between the ice and ence has shown that,as a practical matter, both anhydride is absorbed as latent heat offusion processes are best carried out at a temperature of the ice, sothat the temperature of the solution controlled so that it does notsubstantially exceed is not increased to any appreciable extent. ,This

about C. At such temperatures the esterifiimprovement permits of a moreaccurate concation proceeds more uniformly and the product 30 trol ofthe hydrolysis of the tri-ester, all of which show an improvedfilterability, and a more acis hydrolyzed at substantially the sam'erateand curate control of the hydrolysis of the tri-ester to the sameextent, yielding a homogenous final to a less highly esterified form ispossible so that product having improved properties and suitable thehydrolysis can be terminated precisely when for use in the preparationof articles, such as the acyl content of the ester has reached thedefibers, films, and so forth, of uniform and supesired percentage. riorquality. The hydrolysis may be carried out In this respect, diilicultyis encountered in to any desired extent, depending upon thesolumaintaining the temperature constant due to the bilitycharacteristics desired in the final product. local overheating, andconsequent sharp rise in Usually, in the case of cellulose acetate, forextemperature, which occur when water is added 40 ample, the hydrolysis,is permitted to proceed to the primary acylation solution as a result ofunder the controlled temperature 'conditions'until the exothermicreaction of the water and excess the hydrolyzed ester is soluble inacetone. At anhydride, which reaction is almost instantanethe preferredtemperatures indicated, 1. e., not in ous in the presence of thecatalyst. When Water excess of about 30 C., the hydrolysis proceeds atin the liquid phase is added to the primary acyla- ,a somewhat slowerrate than is the case at tem tion solution, the temperature increase maybe peratures much above 30 C., which has the 'adas great as 20 C. ormore, even when precauvantage that it permits remarkably close and tionsare taken andthe Water is added at an exaccurate control over theprogress of the deacylatremely slow rate. Due to the wide spread in tionof the tri-ester. q the temperature of the primary acetylation solu- Inpracticing the invention, sufiicient ice is tionupon the addition ofwater in the liquid phase added to the primary acylation solution toconthereto, it is impossible to accurately control the vert allexces's'acid anhydride present therein to hydrolysi of the tri-acetateand those portions the corresponding acid, and to reduce the conthereofwhich are overheated are hydrolyzed to a centration of the total acidpresent to about 95% greater extent than the remaining portions, withwith respect to water, after, which the hydrolysis creasing the costthereof.

anhydride or mixtures thereof.

- erally the quantity of catalyst employed in the esterification processis suflicient.

In addition to permitting a more accurate control of the temperature anddegree of deacetylation, thereby resulting in a more homogenous anduniformly deacylated product, the addition of the water in the form ofice for the purpose of killing the excess anhydride and inducinghydrolysis, has the adjunctive advantage that it permits a saving in thetime required for the production of the cellulose esters. Thus,rwhenwater is added to the primary acylation solution considerable time isrequired and restoration of the temperature to the preferred workingrange takes a long time, even when resort is had to the use of externalcooling means, as for instance the circulation of brine through acooling jacket, so

that equipmentand chemicals are tied up for an unduly prolongedperiodand the. time necessary for producing the esters is. extended, therebyin- On the other hand, when ice is added to the primary acetylationsolution in accordance with this invention, only about minutes arerequired for the addition and since the exothermic heat generateddisappears as latent heat of fusion of the ice and the temperature ofthebatch is not substantially, al-

tered, the use of extra external cooling means is unnecessary and timeis. not lost in adjusting the temperature back to the preferred range.Thus, the practice of our invention results in a more economical andtime-conserving method of manufacturing cellulose esters, a saving ofseveral hours being Possible.

The invention may be practiced in connection with the manufacture of anysuitable organic ester. or mixed ester of cellulose, such as celluloseacetate, cellulose propionate, cellulose butyrate,

or cellulose ,acetate-butyrate. The cellulose may be in any suitableform, as cotton, cotton linters,

wood pulp, either sulfite, sulfate, or soda pulp, reconstitutedcellulose, etc. The cellulose may or-maynot beactivated by-preatreatmentwith lower aliphatic acids such as formic or acetic acids, or withalkali, etcvand part .or all of the catalyst to be used in the.esterification may be added tothe pre-treating liquid, if desired. Theesterifying agent will dependon the particular esterwhich it is desiredto produce, and may be acetic anhydride, propionic anhydride, butyricOther.- catalysts besides sulfuric acid may be employed, as forinstance, zinc chloride, sodium bisulfate, methyl Any suitable liquidmay be used as pionicacid, butyric acid, methylene chloride, etc.

For the purpose. of further, describing the inventionthere is given thefollowingexample, in

which the parts are by weight.

Example 100 parts of cellulose are pre-treated'by soaking for 3 hoursin'50 to,100 parts of glacial acetic acid. The pre-treated cellulose isadded to an acetylation medium consisting of 300 parts of aceticanhydride, 300 to 400 partsof glacial acetic acid, and 2,partsofsulfuric acid. The mixture ispermitted toreach a temperature of ,25to30 C., and is maintained within that. range, while being continuouslyagitated, until a homogenous solution is obtained. 60 parts of ice arethenadded, to convert all excess acetic anhydride toacetic acid and toreduce the concentration of the total acetic acid present to 95% withrespect to water. The 95% acetic acid solution of the ester is permittedto stand until the tri-acetate has been hydrolyzed to substantially asecondary acetate as evidenced by the solubility of a sample thereof inacetone, after which the solution is worked up in accordance withcustomary practice.

It will be understood that the foregoing detaileddescription is givenmerely by way of illustration and that modifications may be made thereinwithout departing from the spirit of the invention, which is not to belimited except as defined by the appended claims.

We claim:

1. In the production of cellulose esters wherein cellulose is esterifiedin an excess of a lower fatty acid anhydride in a solvent for the estercontaining a catalyst and at a temperature not substantially in excessof about 30 C., the excess anhydride. present in the primary acylationsolution is converted to the corresponding acid, and the cellulose esterpresent in the solution is hydrolyzed, the method of maintaining thesolution at a temperaturenot substantially in excess of about 30. (3.,during the anhydride conversion and ester hydrolysis, which comprisesadding ice to the primary acylation solution in an amount sufficient toconvert all of the excess anhydride to acid, and to reduce theconcentration of the total acidpresent in the solution to about withrespect to water.

2. In the production of cellulose acetate wherein cellulose isesterified in an excess of acetic anhydride in acetic acid containing-acatalyst, and at atemperature not substantially in excess of about 30C.,' the excess anhydride present in the primary acylation solution isconverted to acetic acid, and the cellulose acetate present in ,thesolution is hydrolyzed, the method: of maintaining the solution at atemperature not sub- .stantiallyin excess of about 30 (3., during theanhydrideconversion and ester hydrolysis, which comprises adding ice tothe-primary acylation solution in an amount sufficient toconvert all oftheexcess acetic anhydride to acetic acid, and to reduce theconcentration of, the total acid present in the solution to about 95%with respect to water.

3. In the production of cellulose esters Wherein cellulose-is esterifiedin an excess of a lower fatty acid anhydride in a solvent for the estercontaining a catalyst, to produce a primary acylation solution, theexcess anhydride present in the primary acylation solution isconverted-to the corresponding acid, and thecellulose ester present inthesolution is hydrolyzed, the method ,of maintaining theprimary'acy-lation solution at ,atemperature not: substantially inexcessof about 30 C., during the anhydride conversion and esterhydrolysis, which comprises adding iceto the primary'acylation solutioninanamount sufficient to convert all of the excess anhydride to acid,and to reduce'the concentration of the total acid present in thesolution to about 95% with respect to water. 7

4. In the production of cellulose-acetate wherein cellulose is.esterified in an excess of acetic anhydride-in acetic acid containing acatalyst to. produce, a primary acylation solution, the excess aceticanhydride present in the primary .acylation'solution isconverted toacetic acid, and the cellulose acetate present in the solution ishydrolyzed, themethod of maintainingthe' pri- I 2,426,982 5 6 maryacylation solution at a temperature not substantially in excess of about30 C., during the REFERENCES CITED anhydride conversion and esterhydrolysis, which h foll win references are of record in the comprisesadding ice to the primary acylation file of this patent: solution in anamount sufiicient to convert all of the excess anhydride to acetic acid,and to reduce 5 UNITED STATES PATENTS the concentration of the totalacid present in Number Name Date the solution to about 95% with respectto water. 2,203,700 Seymour et al. June 11, 1940 ALVAH DALE CLARK.2,007,510 Thornton July 9, 1935 WILLIAM F. REICI-IERT. 1o

